Diagnosis:

Differentiating between a cold and flu may be difficult. Cold symptoms are nearly always less severe than those of the flu.

Diagnosing the Flu

Several available tests can isolate and identify the viruses responsible for some respiratory infections. They are generally not needed, since most cases of the flu are self-evident. However, such tests can be very helpful in confirming or ruling out the flu. If a doctor believes a diagnosis would help, samples using a swab should be taken from the nasal passages or throat within 4 days of the first symptoms.

A nasopharyngeal culture is a test used to identify disease-causing organisms in nasal secretions. Nasopharyngeal cultures are useful in identifying Bordetella pertussis and Neisseria meningitidis (types of bacteria). The culture may help determine appropriate antibiotic therapy.

Several rapid tests for the flu can produce results in less than 30 minutes, but vary on the specific strain or strains that they can detect. They are not as accurate as a viral culture, however, in which the virus is reproduced in the laboratory. Culture results can take 3 - 10 days. Blood tests can also document the infection several weeks after symptoms appear.

Diagnosing Avian Influenza

In February 2006, the U.S. Food and Drug Administration approved a new, faster test for diagnosing H5 strains of avian influenza in people suspected of having the virus. The test is called the Influenza A/H5 (Asian lineage) Virus Real-time RT-PCR Primer and Probe Set. The test gives preliminary results within 4 hours. Older tests required 2 - 3 days. It checks for the presence of the Influenza A H5 strain. If the presence of this strain is confirmed through the rapid test, further testing will be needed to determine the exact subtype of the virus. For example, the current strain of concern is H5, subtype N1, designated as H5N1 for short.

Other Causes of Congestion

Ruling out Allergic Rhinitis. Symptoms of allergic rhinitis include nasal obstruction and congestion, which are similar to the symptoms of a cold. People with allergies, however, are likely to have the following:

• Thin, clear, and runny nasal discharge
• An itchy nose, eyes, or throat
• Recurrent sneezing

There are two forms of allergic rhinitis:

• Symptoms that appear only during allergy season are called allergic rhinitis, commonly known as hay or rose fever. [For more information, see In-Depth Report #77: Allergic rhinitis.]
• Allergens in the house, such as house dust mites, molds, and pet dander, can cause year-long allergic rhinitis, referred to as perennial rhinitis.

Ruling out Sinusitis. The signs and symptoms suggestive of true acute sinusitis include the following:

• A return of congestion and discomfort after initial improvement in a cold (called double sickening)
• Purulent (pus-filled) nasal secretion
• A lack of response to decongestant or antihistamine
• Pain in the upper teeth or pain on one side of the head
• Pain above or below both eyes when leaning over

Children with sinusitis are less likely to have facial pain and headache and may only develop a high fever or prolonged upper respiratory symptoms (such as a daytime cough that does not improve for 11 - 14 days). When the diagnosis is unclear or complications are suspected, further tests may be required. [For more information, see In-Depth Report #62: Sinusitis.]

Other Causes of Coughing

Acute Bronchitis. Acute bronchitis is usually caused by a virus and in most cases is self-limiting. The cough it causes typically lasts for about 7 - 10 days, but in about half of patients, coughing can last for up to 3 weeks, and 25% of patients continue to cough for over 1 month.

Atypical Pneumonia. Pneumonia caused by atypical organisms (for example, Mycoplasma pneumonia, chlamydia, Legionella) can cause symptoms similar to the flu. Only laboratory tests can diagnose the difference. [For more information, see In-Depth Report #64: Pneumonia.]

Ruling out other Viral Infections. Respiratory syncytial virus (RSV) and, possibly human parainfluenza viruses (HPV), are proving to be important causes of serious respiratory infections in infants, the elderly, and people with damaged immune systems. (Both also cause mild conditions.) RSV may be a much more common cause of flu-like symptoms than previously thought.

Pertussis. Pertussis (whooping cough) was a very common childhood illness throughout the first half of the century. Although immunizations caused a decline in cases to only 1,700 in the U.S. in 1980, the incidence has risen recently, with 8,739 cases in 2007. Many more cases are reported worldwide.

Nearly half of pertussis cases now occur in people 10 years of age or older, perhaps due to waning immunity in adolescents and adults. Such cases may be greatly underreported. Up to 25% of adults who see a doctor for persistent cough may actually have pertussis. It may go undiagnosed, however, because their symptoms are usually mild, and adults are unlikely to have the classic "whooping" cough. This is of some concern because such adults may unknowingly infect unvaccinated children. The younger the patient, the higher the risk for severe complications, including pneumonia, seizures, and even death. Children younger than 6 months are at particular risk because protection is incomplete, even with vaccination.

In April 2005, the FDA approved the first pertussis-containing booster vaccine ("Boostrix") for children aged 10 - 18. In December 2008, the FDA approved this vaccine for use in adults up to age 64.

Other Causes of Sore Throat

In addition to common cold viruses, other, less frequent causes of sore throat include the following:

• Strep throat
• Foodborne and waterborne infections (Streptococcus C and G)
• An uncommon organism called Arcanobacterium haemolyticum (infection with this bacterium can mimic strep throat and may even cause a rash)
• Infectious mononucleosis ("mono")
• Herpesvirus 1

What is Strep Throat?

Group A Streptococcal bacteria is the most common bacterial cause of the severe sore throat known commonly as "strep throat." It occurs mostly in school age children, but people of all ages are susceptible. (Strep throat constitutes about 12% of all sore throat cases seen by doctors.)

The symptoms of strep throat include the following:

• A sudden onset of severe sore throat
• Difficulty in swallowing
• Fever
• Headache
• Stomach pain
• Vomiting

Only about half of patients with strep throat have such clear-cut symptoms. Furthermore, half of people who have these symptoms do not actually have strep throat.

How Is Strep Throat Diagnosed? Most cold-related sore throats are caused by viruses and require no treatment. They usually do not last more than a day. When the sore throat persists and is very painful the doctor will want to rule out or confirm the presence of the Streptococcus bacteria.

• The doctor will look for redness and pus-filled patches on the tonsils and back of the throat. Enlarged tonsils are less likely to indicate a strep throat.
• The doctor will feel the sides of the neck for swollen lymph nodes. If the lymph nodes are not swollen, it is less likely to be a strep throat.
• A cotton swab is used to take a sample of pus in the throat for a throat culture.

A throat culture is the most effective and least expensive test for confirming the presence of strep throat. It takes 24 - 48 hours to obtain a result.

Rapid Antigen-Detection Test for Strep Throat. A faster test, called the rapid strep antigen test, uses chemicals to detect the presence of bacteria in a few minutes. A positive result nearly always means that streptococcal bacteria are the cause of the infection. The test, however, fails to detect 5 - 10% of cases, so a culture may still be necessary to catch any missed infections, particularly in children.

How Serious is Strep Throat? The use of antibiotics has removed the threat of most complications from streptococcus infection in the throat. However, untreated strep throat could lead to the following complications:

• Abscess in the tonsils
• Scarlet fever
• Rheumatic fever (rare in the U.S.)

How Is Strep Throat Treated? Strep throat infections require antibiotics. Antibiotics prevent a serious complication called rheumatic fever, which can result in permanent damage to the heart. Fortunately, this complication rarely occurs in United States anymore. If started on time, antibiotic treatment of strep throat will almost always prevent this complication. In addition, antibiotics shorten the recovery time from strep throat.

The following antibiotics are generally used to treat strep throat:

• Penicillin is usually the antibiotic of choice unless the patient is allergic to it. A full 10 days of treatment may be necessary to clear the infection. Amoxicillin, a form of penicillin, is proving to be effective when taken in a single daily dose for 10 days.
• Macrolide antibiotics. Erythromycin is known as a macrolide antibiotic and is the first choice for patients with penicillin allergies. A 10-day regimen is needed to clear the infection. Another macrolide, azithromycin, can be given as a single daily dose and may be effective in 5 days. It is expensive, however, and bacterial resistance to macrolides is growing, so it should not be given as a first choice.
• Cephalosporins are very effective in eradicating the bacteria.

Antibiotics are very often inappropriately prescribed for non-strep sore throats. Studies indicate that fewer than half of adults and far fewer of the children with even strong signs and symptoms for strep throat actually have strep infections.

Parents should be comforted that a delay in antibiotic treatment while waiting for lab results does not increase the risk that the child will develop serious long-term complications, including acute rheumatic fever. If a patient is severely ill, however, it is reasonable to begin administering antibiotics before the results are back. If the culture is negative (there is no evidence of bacteria), the doctor should call the family to make certain the patient stops taking the antibiotics and any remaining pills are discarded.

Children who have a sore throat and who have had rheumatic fever in the past should receive antibiotics immediately, even before culture results are back. Children with a sore throat who have a family member with strep throat or rheumatic fever should also receive immediate antibiotic treatment.